The present invention relates in general to communication systems and components, and is particularly directed to a transconductance amplifier circuit, that is configured to transform a single ended input voltage into a very precise, single ended output current, in a manner that is effectively independent of respective voltage supply rails through which the amplifier circuit is powered, and which can be operated at a very low quiescent current. As a non-limiting example, the invention may be readily employed to implement various circuit blocks of a subscriber line interface circuit, enabling it to enjoy substantially reduced power requirements for interfacing communication signals with a telecommunication wireline pair.
A wide variety of electronic circuit applications employ one or more transconductance stages to generate output/drive currents that can be reasonably accurately controlled for delivery to one or more loads. As a non-limiting example, various equipments employed by telecommunication service providers contain what are known as xe2x80x98SLICxe2x80x99s (subscriber line interface circuits), to interface (transmit and receive) telecommunication signals with respect to (tip and ring leads of) a (copper) wireline pair.
Because the length of the wireline pair can be expected to vary from installation to installation, may have a very significant length (e.g., on the order of multiple miles), and transports both substantial DC voltages, as well as AC signals (e.g., voice and/or ringing), designing a SLIC that has xe2x80x98universalxe2x80x99 use in both legacy and state of the art installations continues to be a daunting task for the circuit designer.
In order to accommodate the above-referenced parameter variations in a telecommunication signalling environment, it is customary practice to configure the SLIC as a transconductance amplifier-based circuit, that produces a prescribed output current in response to an input voltage.
One of the issues involved in using a transconductance amplifier circuit is the fact that it must not only deliver a very precisely controlled output current, but must do so irrespective of the voltages of the supply rails from which it is powered.
Conventional transconductance amplifier stages, whether they involve single ended implementations or differentially coupled transistor pairs (such as that shown at Q1-Q2 in FIG. 1), usually suffer from the presence of one or more non-linearities associated with unequal or mismatched diode junctions in the components of the circuit generating a single ended output current.
One way to obviate this problem would be to employ a differentially balanced operational amplifier circuit architecture, such as that illustrated diagrammatically in FIG. 2. As shown therein, a pair of operational amplifiers A1 and A2 may be coupled to respective drive inputs (bases) of a pair of transistors Q1-Q2. Transistors Q1 and Q2 have their output (collector-emitter) current flow paths coupled in a differential configuration between a current mirror circuit M and negative feedback paths of the amplifiers A1 and A2, which terminate opposite ends of an impedance (resistance) Z. Although this dual amplifier circuit design enables an output current to be precisely generated in terms of an applied input voltage, it does so at an increase in complexity and therefore device count, power and cost, and is constrained by the large signal bandwidth limitations of the operational amplifiers.
In accordance with the present invention, shortcomings of conventional transconductance amplifier circuits, such as those discussed above, are effectively obviated by a new and improved transconductance amplifier circuit, that is operative to transform a single ended input voltage (which may be a composite of plural input voltages) into a very precise, single ended output current, without requiring a substantial quiescent current, and in a manner which is effectively independent of (differential) voltage supply rails through which the circuit is powered.
For this purpose, the transconductance amplifier circuit of the invention includes an operational amplifier configured as a single ended, unity gain buffer, having a high input impedance, moderate voltage gain, dual polarity input stage, and a low output impedance, single ended output stage. The input stage has its non-inverting polarity input referenced to a DC reference voltage (which may be signal ground), and its inverting polarity input coupled over a negative feedback path to an input/output node of the output stage. The output stage is configured as a DC biased, differentially coupled transistor buffer circuit pair.
Unlike a conventional amplifier circuit, the input/output node of the output stage, rather than being employed to supply an output current to a downstream load, is employed as an input node and is adapted to receive one or more input currents coupled via one or more coupling resistors from associated input voltage feed ports. Also, series-connected current paths through output transistors of the differentially coupled output stage buffer circuit transistor pair, rather than being powered directly by respective voltage supply rails (e.g., Vcc and Vee), are coupled in circuit with first current supply paths of associated current mirror circuits, which serve to isolate the biasing of the amplifier""s output stage from the power supply rails. Second current supply paths of the current mirror circuits are coupled to the single ended output port of the transconductance amplifier circuit.
The relationships among the currents through the two current supply paths of the mirror circuits and the input/output node of the output stage of the transconductance amplifier of the present invention are such that the output current produced at the single ended output port is linearly proportional to the (composite) input current appearing at the input/output node of the output stage. In addition, if the time average value of each of the input voltages applied to the voltage input terminals is equal to the reference voltage applied to the non-inverting input of the operational amplifier and that reference voltage is a DC voltage, then the time averages of the mirrored currents supplied to the output stage are proportional to the DC bias current flowing in the output stage. As a consequence, if the value of the DC bias current is small and the current mirror ratio K is equal to or less than one, the quiescent power can be reduced to an extremely low value.
One application of the transconductance amplifier circuit of the present invention is as a building block for one or more subcircuits employed within a subscriber line interface circuit, used to interface communication signals supplied from a device, such as a modem, with a tip and ring ports of a wireline pair for transport to a remote circuit, such as a subscriber""s telephone. In the case of a receiver channel circuit, as a non-limiting example, the transconductance amplifier of the invention may be augmented by a pair of auxiliary current mirror circuits cross-coupled with the above-referenced current mirror circuits of the transconductance circuit. The current relationships associated with the cross-coupling of the auxiliary current mirror circuits with those of the transconductance amplifier circuit are such that the auxiliary current mirror circuits supply the same precision output current (but in an opposite directional sense) at an additional output port. These two (opposite polarity) current output ports are applied through respective output amplifiers to tip and ring output ports for application to a telephone wireline pair.
The input/output node of the transconductance amplifier is coupled through respective input resistors to receive a plurality of input channel voltages. These input channels may include a received channel signal, a feedback channel voltage (which may be obtained by sensing a tip-ring output voltage for synthesizing an appropriate value of a termination impedance between the tip and ring terminals), a teletax or pulse metering signal, and a ringing signal voltage. The voltage gain for each input signal can be appropriately tailored by the selected value of its input resistor.
The DC output voltages at the tip and ring ports are determined by DC bias currents supplied by tip and ring DC bias current sources multiplied by the values of feedback resistors between the tip and ring ports and inputs of the tip and ring output amplifiers. These DC bias currents provide an overhead voltage for the sum of all of the input voltages applied to the input/output port of the transconductance amplifier, as well as a sufficient differential DC voltage between the tip and ring terminals to supply the necessary DC current required to bias the phone at the far end of the loop, which may be more than several miles away.